1. Field of the Invention
The invention relates to a semiconductor wafer composed of silicon, which contains fluorine and is particularly suitable for producing electronic components because it has no agglomerated intrinsic point defects rated as disturbing. The invention also relates to a method for producing such a semiconductor wafer.
2. Background Art
Since the beginnings of microelectronics the manufacturers of semiconductor wafers composed of silicon have devoted themselves particularly to the task of eliminating defects rated as disturbing, at least in that region of the semiconductor wafer in which the construction of structures of electronic components is provided. Defects attributable to intrinsic point defects (silicon interstitials and vacancies) are generally rated as disturbing when their largest spatial extent in one direction is comparable to the extent of the smallest component structures or larger. Defects of agglomerated silicon interstitials which are surrounded by a network of dislocation loops and which have an extent in the micrometer range are referred to as A defects or Lpit defects (“large etch pit defects”). These defects are normally not acceptable at all. Defects which are formed by agglomerated vacancies and can be detected, for example as COP defects (“crystal originated particles”) by means of infrared laser scattered light tomography, are tolerated only as long as they are smaller than the smallest component structures.
According to Voronkov's theory, the validity of which is supported by experimental results, the concentration of point defects in the single crystal is essentially determined by the ratio V/G during crystallization of the melt, where V denotes the rate of the crystallization and G denotes the axial temperature gradient at the boundary between the melt and the growing single crystal. According to the theory, there is a specific ratio V/G at which the concentrations of silicon interstitials and vacancies are identical. Below this ratio, interstitials are present in excess, and above the ratio, vacancies are present in excess. If a dominant type of point defect attains supersaturation during cooling, agglomerates of this type of point defect can arise. Agglomerates of vacancies (“voids”) become increasingly larger, the longer the duration during cooling within the range of formation temperature of agglomerates. One strategy for avoiding disturbing vacancy agglomerates therefore consists in providing for the shortest possible residence time in the range of the formation temperature, which is around 1100° C., during cooling. However, single crystals having diameters of 200 mm or greater cannot be cooled arbitrarily rapidly, with the result that limits are imposed on controlling the size of defects by limiting the residence duration in the case of such single crystals. Another strategy pursues the aim of controlling the V/G ratio as far as possible such that a significant excess of one type of point defect does not actually arise in the first place. Such control is very complex in terms of process technology and not very economical because a comparatively low rate V has to be employed in this case. The process window available, that is to say the range of values within which the ratio V/G must be controlled, is narrowly delimited, moreover. For a predetermined G, the pulling rate V is normally permitted to vary only by ±0.01 mm/min. This latter requirement generally has the effect that parts of the single crystal cannot be utilized because they have defects that cannot be tolerated.
US2003/0008479 A1 describes a method for producing single crystals composed of silicon according to the Czochralski method, in which the presence of a “halogen getter” in the melt leads to a purifying of the melt, which brings about an attenuation of metal-induced defects, COP defects and other defects. In the preferred configuration, the melt contains at least 0.01% by weight or at least 4·1018 atoms/cm3 of the “halogen getter”.